Read the passage. The Northmen came to Paris with 700 sailing ships. This is not counting those of smaller size which are commonly called barques. At one stretch the Seine was lined with the vessels for more than two leagues. You could ask in astonishment in what cavern the river had been swallowed up. There were so many boats the river could not to be seen.

–Adapted from "Wars of Count Odo with the Northmen” by Abbo

What is the central idea of the passage?